1. Field of the Invention
This invention relates generally to the field of electromagnetic (EM) well logging. More particularly, the invention concerns devices for reducing and/or correcting for the effects of the borehole on an overall subsurface formation measurement.
2. Background Art
Induction and propagation logging techniques have been employed in hydrocarbon and water exploration and production operations for many years to measure the electrical conductivity (or its inverse, resistivity) of subsurface formations. These techniques entail the deployment of antennas into a borehole to emit EM energy through the borehole fluid (also referred to herein as mud) and into the subsurface formations. Conventional logging techniques include “wireline” logging, logging-while-drilling (LWD), and logging-while-tripping (LWT). Wireline logging entails lowering the antennas into the borehole on a “sonde” or support at the end of an electrical cable to obtain the subsurface measurements as the instrument is moved along the borehole. LWD entails mounting the antennas on a support connected to a drilling assembly to obtain the measurements while a borehole is being drilled through the formations. LWT involves placing a support equipped with antennas near the bottom of the drill string and making measurements while the string is withdrawn from the borehole.
Conventional antennas are formed from coils of the cylindrical solenoid type comprised of one or more turns of insulated conductor wire wound around a support. These antennas are typically operable as sources and/or sensors. In operation, a transmitter antenna is energized by an alternating current to emit EM energy. The emitted energy interacts with the mud and the formation, producing signals that are detected and measured by one or more of the antennas. The detected signals are usually expressed as a complex number (phasor voltage) and reflect the interaction with the mud and the formation. By processing the detected signal data, a profile of the formation and/or borehole properties is determined.
A coil carrying a current can be represented as a magnetic dipole having a magnetic moment proportional to the current and the area encompassed by the coil. The direction and strength of the magnetic dipole moment can be represented by a vector perpendicular to the area encompassed by the coil. In conventional induction and propagation logging systems, the antennas are typically mounted on a metallic “sonde” or support with their axes along the longitudinal axis of the support. Thus, these instruments are implemented with antennas having longitudinal magnetic dipoles (LMD). U.S. Pat. No. 4,651,101 describes a logging sonde implemented with LMD antennas. When such an antenna is placed in a borehole and energized to transmit EM energy, currents flow around the antenna in the borehole and in the surrounding formation. There is no net current flow up or down the borehole.
An emerging technique in the field of well logging is the use of instruments incorporating antennas having tilted or transverse coils, i.e., where the coil's axis is not parallel to the support axis. An antenna with its axis perpendicular to the support axis is usually referred to as a transverse antenna. These instruments are thus implemented with antennas having a transverse or tilted magnetic dipole (TMD). One particular implementation uses a set of three coils having non-parallel axes (referred to herein as tri-axial). The aim of these TMD configurations is to provide EM measurements with directional sensitivity to the formation properties. Transverse magnetic fields are also useful for the implementation of nuclear magnetic resonance based methods. U.S. Pat. No. 5,602,557, for example, describes an arrangement that has a pair of “saddle-coil” conductor loops lying opposite one another and rotationally offset 90° relative to one another. Other instruments equipped with TMDs are described in U.S. Pat. Nos. 6,163,155, 6,147,496, 5,757,191, 5,115,198, 4,319,191, 5,508,616, 5,757,191, 5,781,436, 6,044,325, 4,264,862 and 6,147,496.
If a transmitter is placed in a homogeneous medium, currents will flow in paths surrounding the transmitter. When a borehole is added, these current paths are distorted. These currents induce a voltage in a receiver displaced from the transmitter. This voltage is an indication of the resistivity of the formation. If instead of a homogeneous medium, we include a borehole, then the current paths are altered and hence the received voltage is different from what would be measured in the absence of a borehole. This difference is called the “borehole effect.” The difference in borehole effect between a LMD-based tool and a TMD-based tool is due to the difference between the distortion of the currents in the presence of a borehole.
A particularly troublesome property of the TMD is the extremely large borehole effect that occurs in high contrast situations, i.e., when the mud in the borehole is more conductive than the formation. When a TMD is placed in the center of a borehole, there is no net current along the borehole axis. When it is eccentered in a direction parallel to the direction of the magnetic moment, the symmetry of the situation insures that there is still no net current along the borehole axis. However, when a TMD is eccentered in a direction perpendicular to the direction of the magnetic moment, axial currents are induced in the borehole. In high contrast situations these currents can flow for a very long distance along the borehole. When these currents pass by TMD receivers, they can cause signals that are many times larger than would appear in a homogeneous formation without a borehole, resulting in erroneous measurements.
U.S. Pat. No. 4,319,191 (assigned to the present assignee) describes a sensor assembly aimed at protecting a solenoid from the borehole environment. U.S. Pat. No. 5,041,975 (assigned to the present assignee) describes a technique for processing signal data from well logging measurements in an effort to correct for the effects of the borehole. U.S. Pat. No. 5,058,077 describes a technique for processing downhole sensor data in an effort to compensate for the effect of eccentric rotation on the sensor while drilling. U.S. Pat. No. 5,781,436 describes a technique for measuring the conductivity of earth formations by making subsurface EM measurements at multiple frequencies and preselected amplitudes. However, none of these patents relates to the properties or effects of TMDs in subsurface measurements.
U.S. Pat. No. 6,667,620 (assigned to the present assignee) describes an insulating sleeve arrangement slideably coupled to cover a sensor assembly containing TMDs from direct contact with the borehole. The sleeve includes a series of electrically conductive elements in alignment with the longitudinal axis of the tool body. The conductive elements are connected to the internal tool body via a conductor impregnated within the sleeve material. The mechanical design of the impregnated conductors may allow the risk of fluid leakage of oil from under the sleeve to the borehole or water or other borehole fluid through the sleeve to the sensor assembly or tool support. Furthermore, manufacturing costs of impregnating conductors within the sleeve warrant consideration of less expensive alternatives.
Thus there remains a need for improved methods and apparatus for handling the flow of undesired axial currents along the borehole.